US President Barack Obama urged Americans to honour emergency service workers and men and women in uniform who keep the country safe as he marked the 11th anniversary of the September 11, 2001 attacks.
"It's a chance to honour the courage of the first responders who risked their lives - on that day, and every day since," Obama said in his weekly radio and internet address.
"And it's an opportunity to give thanks for our men and women in uniform who have served and sacrificed, sometimes far from home, to keep our country safe."
Memories remain raw of the day when al-Qaeda hijackers slammed two passenger planes into the World Trade Centre in New York, destroying its iconic Twin Towers, and a third into the Pentagon building, in the nation's capital.
A fourth plane crashed in a field when the passengers overpowered the hijackers.
Almost 3,000 people were killed that day in the worst attacks on American soil.
Obama said the attacks 11 years ago filled Americans with questions about the origins of terrorism and how the United States should respond to it.
"The last decade has been a difficult one, but together we have answered those questions and come back stronger as a nation."
The president said the US had now decimated al-Qaeda's leadership and put the terror network on a path to defeat.
"And thanks to the courage and skill of our intelligence personnel and armed forces, Osama bin Laden will never threaten America again."